Replace usages of the terms whitelist and blacklist with the more
inclusive and more precise terms allowlist and blocklist, which are
actually also more consistent with parts of the codebase, e.g.
mct_app_filter_is_content_type_allowed().

The only API break here is in libmalcontent/app-filter.h but the
relevant API is not used anywhere else in Endless OS beyond this repo,
nor to my knowledge in any other distribution. Also, per the README,
this project's API is not stable, so now is a good time to make this
change.